Chemistry Reference
In-Depth Information
experimental evidence is lacking. On p. 1601 in Ref. [
1
]J ? S say: ''Qualitatively,
as would be expected, the yield of rings in a polymer system is predicted to
increase with dilution. A less trivial and more interesting feature of the theory rests
in the mathematical coincidence of the ring distribution equation with those
expressing the thermodynamic properties of the perfect Bose-Einstein gas. Thus, a
critical phenomenon arises which is normally analogous to the well known B-E
condensation: there is a critical concentration, below which the condensing system
can be converted into rings, but above which this is not possible [
16
]''.
Unfortunately, neither Stockmayer nor Flory contributed any study to this
problem. Despite the numerous studies of equilibrated PDMS also Semlyen did not
discuss this point. Since equilibration of cyclic monomers or very long chains are
equivalent to a TC polycondensation with high conversion ([ 99.9 %), results
obtained from equilibration of cycles above the critical concentration are of great
interest with regard to the crIMC. About the KOH catalyzed equilibration of
cyclotetrasiloxane, first reported by Brown et al. [
62
], Semlyen made the following
comment [
74
]: ''The last authors reported large scale ring-chain equilibration
reaction for PDMS in solution in toluene (at a siloxane concentration of 222 g/L)
and in bulk. In the 5 L solution equilibrate cycles are present as 95 % of the total
siloxane, so that the chains constitute about 1 % of the solution as a whole''. The
molar (SiMe
2
-O-) concentration corresponded to 3 mol/L, but Semlyen did not
say, if this high concentration was below or above the critical concentration. A
recent calculation of Szymanski showed [
92
] that 3 mol/L is very close to the
crIMC, but not above. For the equilibration in bulk no information about the
content of cycles was given. Carmichael [
59
] mentioned in a review article that the
fraction of cyclosiloxane in PDMS equilibrated in bulk amounts to 15 %. How-
ever, his analytical method did not allow for a quantification of higher cyclic
oligomers and polymers, and thus, his data are meaningless.
L½
p
¼ L½
0
1
p
ð
Þ
with
L½
¼molar concentration of all linear species
ð
5
:
25
Þ
At this point the afore-mentioned results of Kricheldorf et al. [
46
,
47
] con-
cerning cyclic polyamides come into play. These syntheses of Nylons were con-
ducted in bulk at an IMC, around 10 mol/L, corresponding to the conditions used
by J ? S for the calculation of the lowest curve in Fig.
5.1
(calculated with B
0
/
c = 0.005) and in perfect agreement with the conditions used by Flory for his
calculations (2.5 weight % of cycles) presented in
Sect. 4.3
. After optimization of
the conversion, around 30 % of the virgin reaction products were removed by
repeated extraction and reprecipitation. Furthermore, the MALDI-TOF spectra
evidenced that the insoluble remainder still contained a large fraction of cyclic
polyamides. This means that the weight fraction of cyclic species was at least by a
factor 10 more likely by a factor 20 higher than calculated by J ? S and Flory for
an aliphatic a
2
? b
2
polycondensate. Moreover, the best mass spectrum of Nylon-
6 exclusively displayed peaks of cyclics upto masses around 11 000 Da corre-
sponding to a DP (x) around 100. These results are far outside any margin of error
